1. Field of the Invention
The invention relates to universal couplings and automotive half-shafts, and more particularly, to constant-velocity universal joints for directly connecting two shafts in a manner that transmits rotation from the driving shaft to the driven shaft while, at the same time, permitting the angle of intersection between the axes of the shafts to be varied away from 180° alignment in any direction over a relatively wide and continuous range of angles (e.g., 60° or more).
2. Description of Related Art
There are well-known, non-gear means for transmitting rotary motion between shafts experiencing angular change. Perhaps the best known of such devices are the universal joints used to connect the drive shafts and wheel axles of automotive vehicles. Such universal joints are often constructed in the venerable double-yoke (Cardan) form of two small intersecting axles interconnected by a pair of yokes. However, the shafts connected by such yoke and axle joints do not turn at the same rate of rotation throughout each entire revolution. Therefore, constant velocity (“CV”) joints have been developed (e.g., Rzeppa and Birfield), in which the points of connection between the angled shafts are provided by sliding balls, which, during each revolution of the driving and driven shafts, slide back and forth in individual tracks to maintain their respective centers at all times in a plane which bisects the instantaneous angle formed between the shafts. However, such universal and CV-joints are quite complex and relatively difficult to lubricate, and the design and manufacture of such joint components is widely recognized as a very specialized and esoteric art of critical importance to the worldwide automotive industry. While this universal joint art is very well developed, the joints are expensive, including many parts that are difficult and expensive to manufacture due to large surface areas that must be ground with extreme accuracy (e.g., ±0.0002″/0.005 mm). Such joints are limited in regard to the rotational speeds that they can transmit and, more particularly, in regard to the size of the angles over which they can operate efficiently.
In the widely used Rzeppa CV-joint design, for example, with every rotation of the joint there is: (a) considerable reciprocating sliding action along both internal and external meridional (curved longitudinal) ball guide slots, as well as (b) an additional crosswise sliding action of the balls across the rectangular slots of the required spherical ball retainer; (c) sliding of the spherical inner race required by these designs against the male spherical surface of the housing cup as well as against the male spherical diameter of the slotted core element. The frequency of these sliding actions produces heat that increases in proportion to operating speeds and shaft angles. Further, the Rzeppa joint designs also necessitate camming modifications to both inner and outer meridional ball-guide slots in order to force the balls and their retainer into a constant-velocity plane position. These cam angles also guarantee that a portion of the ball motion along the slots occurs as a sliding, rather than a pure rolling, motion.
With respect to motion limitation in the existing commercial CV-joint designs, the funnel angle (or combined inner and outer cam angles) of Rzeppa meridional slots needs to be higher than 15° to avoid ball-jamming friction, and thus, respective inner and outer ball-guide slots converge and diverge rather rapidly, limiting the total angular range that can be accommodated in a reasonably-sized CV-joint assembly package.
A universal coupling using a new type of “spherical” gearing was disclosed in U.S. Pat. No. 5,613,914. That patent, and its many corresponding patents throughout the world, disclosed spherical gears having several different possible tooth forms that could be incorporated into various designs of disclosed CV-joints. This spherical gearing is based on a radically different gear geometry design. Namely, the use of a single pair of gears to transmit constant velocity between two shafts is accomplished by a design in which one of the gears has internal teeth and the other has external teeth. The pitch circles of the two gears are of identical size and always remain, in effect, as great circles on the same pitch sphere. As is axiomatic in spherical geometry, such great circles intersect at two points, and the pair of lunes formed on the surface of the sphere between the intersecting great circles (i.e., between the pitch circles of the two gears) inscribe a giant lemniscate (“figure-eight”) around the surface of the sphere. Since the relative movement of the tooth contact points shared between the mating gears inscribe respective lemniscates at all relative angular adjustments of the gear shafts, the two shafts rotate at constant velocity.
Although the pitch circles of each spherical gear have just been indicated to be theoretical great circles on the same pitch sphere, it may be easier to conceptualize such spherical gearing by thinking of each gear of the pair as having its own respective theoretical pitch surface, thereby permitting the necessary relative motion between the gears. Thus, each spherical gear may also be thought of theoretically as having its own respective pitch surface in the form of a respective one of a pair of respective pitch spheres that have coincident centers and radii which are substantially identical while permitting each pitch sphere to rotate independently about its respective axis. Therefore, each pitch circle can also be considered theoretically to be, respectively, a great circle on a respective one of these substantially identical pitch spheres so that the pitch circles of the gear pair effectively intersect with each other at two points separated by 180° (i.e., “poles”), and the axes of rotation of the two respective pitch spheres intersect at the coincident centers of the two pitch spheres at all times and at all angles of intersection.
A pair of full-sized steel gears was built, and bench tested, clearly validating that spherical gearing is capable of providing substantially true constant velocity with low friction for angular connections when operating at high speeds while the angles between the shafts are continuously varying through a wide range of angles, e.g., a much wider range of angles than presently achieved by standard commercial automotive CV-joints. Unfortunately, the spherical gearing disclosed in U.S. Pat. No. 5,613,914 is fairly complex, difficult to manufacture, and lacks the practicality required for commercial CV-joint use.
Universal joints are presently used in the forms of (a) interlocking yokes (e.g., Cardan joints) to provide angular interconnections in the drive shafts of vehicles and (b) automotive half-shaft drive axles to connect the output shafts of drive differentials with the turning and bouncing drive wheels of a vehicle. A typical commercial half-shaft includes two different types of universal joints, e.g., a Rzeppa universal joint at one end and a tri-pot universal joint at the other end. Each of these joints is complex and expensive to manufacture. The Rzeppa universal joint uses six precision ground balls that, as just indicated above, slide back and forth in a complex of respective precision ground tracks, and the tri-pot universal joint uses three precision ground spherical rollers and straight ground tracks.